Method of controlling a wind turbine

ABSTRACT

A method of operating a power generating system for a wind turbine connected to an electrical grid, the power generating system comprising a power generator, a converter, a transformer and a tap changer, the method comprising; monitoring a signal for detecting a voltage in the electrical grid which requires an increase in output voltage from the power generating system; determining a partial-load condition of the converter, which corresponds to the converter being configured to output a voltage which is substantially below its maximum output voltage; and upon determining the partial-load condition, operating the tap changer to tap down the transformer, and operating the converter to provide the required increase in output voltage from the power generating system.

TECHNICAL FIELD

The invention relates to wind turbines, particularly to the control ofpower generating systems of wind turbines.

BACKGROUND

A wind turbine typically includes a tower, a generator, a gearbox, anacelle, and a rotor having one or more rotor blades. The rotor bladestransform wind energy into a mechanical rotational torque that drivesone or more generators via the rotor. The generators are rotationallycoupled to the rotor through the gearbox. A power converter with aregulated DC link controlled by a converter controller is furtherprovided to convert a frequency of generated electric power to afrequency substantially similar to a grid frequency.

Renewable energy power generator systems, such as the wind turbinedescribed above, are typically operated within predetermined voltage andpower factor tolerance ranges. The operational tolerance ranges enablethe wind turbine to supply a reliable transmission of electrical powerto the grid over a variety of operating conditions and to provideancillary functions, such as the injection and absorption of reactivepower, in order to support grid stability. For example, a grid voltagetolerance range may extend from 90% to 110% of the nominally ratedvoltage whilst a typical electrical grid power factor tolerance rangeextends from +0.9 to −0.9 power factor (pf). These operational toleranceranges define the electrical parameters for all components connected tothe grid including the current rating and power draw for voltages in thelower end of the voltage range and the voltages at the upper end of thevoltage range.

It is known to configure the grid connected wind turbines so that theyoperate within a voltage and/or power factor range that is complimentaryto the operational tolerance ranges associated with the grid. Inparticular, each wind turbine is arranged to operate within apre-determined range of parameter values, which includes powergeneration, current, voltage, and power factor.

In order to accommodate the potential large voltage transients in theelectrical power grid, each of the ‘complimentary’ wind turbines may beoperated for prolonged periods below their upper power and currentparameters. Operating the wind turbines in this way reduces theiroperating effectiveness and efficiency, leading to an underutilisationof latent wind energy resource, whilst also incurring potential economiclosses for the owners/operators of the wind farms.

Furthermore, for a typical wind turbine its operating parameters arelimited by a maximum voltage for one or more of the components of thepower generating system. For example, a wind turbine may be required toprovide reactive power to the grid, which would impose over-voltageconditions on transformer windings where the converter is connected.Thus, when the converter provides reactive power, the resulting voltagemay exceed a maximum specified continuous operating voltage level. Inorder to mitigate such over-voltage conditions, the converter can beoperated to shift the power factor away from the required value;however, this is not always optimal.

Accordingly, there is need to improve systems and methods of optimisingwind turbine operation while also maintaining voltage levels withinspecified operating ranges. It is against this background to which thepresent invention is set.

SUMMARY

According to a first aspect of the invention there is provided a methodof operating a power generating system for a wind turbine connected toan electrical grid, the power generating system comprising a powergenerator, a converter, a transformer and a tap changer, the methodcomprising; monitoring a signal for detecting a voltage in theelectrical grid which requires an increase in output voltage from thepower generating system; determining a partial-load condition of theconverter, which corresponds to the converter being configured to outputa voltage which is substantially below its maximum output voltage; andupon determining the partial-load condition, operating the tap changerto tap down the transformer, and operating the converter to provide therequired increase in output voltage from the power generating system.

By tapping down the tap changer when in a partial load condition,control method advantageously increases the potential over voltage ridethrough (OVRT) capabilities of the power generating system compared to asystem which does not include a tap changer. This is because the windturbine generator will typically only operate at full load for a portionof its operational life, and the rest of the time it will operate in apartial load condition. Accordingly, the tapping down the transformerduring partial load conditions, results in an increase in a converteroperating current, which thereby reduces the associated converter lossesfor the majority of the converter's operational life.

Operating the tap changer may comprise: determining a minimum tapchanger adjustment which will still allow the converter to provide therequired increase in output voltage; and operating the tap changer totap down the transformer to implement the minimum tap changeradjustment.

The method may comprise: monitoring a signal for detecting the speed ofthe wind in the area of the wind turbine; and determining an operatingcondition of the power generating system based on the detected windspeed.

The method may comprise; determining that the power generating system isoperating in a first operating condition; monitoring a signal fordetecting an operating current I_(C) of the converter; and upondetermining that the operating current I_(C) is above a thresholdcurrent I_(T), inhibiting the tap down of the transformer.

The threshold current I_(T) may be 1.0 per unit system.

The first operating condition may correspond to the wind speed beingbetween 0 m/s and 10 m/s.

The method may comprise; determining that the power generating system isoperating in a second operating condition; and operating the tap changerto limit the magnitude of the tap-down of the transformer based on thedetected wind speed.

The second operating condition may correspond to a wind speed of between10 m/s and 11 m/s.

The method may comprise disabling the method of operating the powergenerating system upon determining that the wind speed is above apre-determined wind threshold.

The power generating system may be configured to provide an over-voltageto the electrical grid; the method may comprise, upon determining thatthe power generating system is no longer required to provide anovervoltage to the electrical grid, operating the converter and the tapchanger according to a pre-determined operating protocol.

Monitoring the signal for detecting a voltage in the electrical gridwhich requires an increase in output voltage from the power generatingsystem may comprise;

-   -   monitoring a first signal indicative of a tap position of the        tap changer;    -   monitoring a second signal indicative of a voltage at a        low-voltage side of the transformer; and    -   determining the voltage of a high-voltage side of the        transformer based on the first and second signal.

Determining the voltage at the high-voltage side of the transformer maycomprises:

$U_{{LV} - {virtual}} = {\frac{n_{2} + {{\Delta n}*N_{TC}}}{n_{2}}U_{LV}}$

-   -   wherein U_(LV-virtual) defines a virtual voltage at the        low-voltage side of the transformer when the tap changer is        configured in a neutral position, n₂ is a number of windings at        a high-voltage side of the transformer, N_(TC) is the tap        position of the tap changer, A n is a change in the number of        windings at the high-voltage side for a given tap position, and        U_(LV) defines the actual voltage at the low-voltage side of the        transformer; wherein an over-voltage condition is detected if        both the virtual voltage U_(LV-virtual) and the actual voltage        U_(LV) are determined to be within an over-voltage range.

Determining the voltage at the high-voltage side of the transformer maycomprise:

$U_{{HV} - {estimate}} = {{\frac{n_{2} + {{\Delta n}*N_{TC}}}{n_{1}}U_{LV}} + {I_{reactive}*X_{HV}}}$

-   -   wherein U_(HV-estimate) defines an estimated voltage at the        high-voltage side of the transformer when the tap changer is        configured in a neutral position, n₂ is a number of windings at        a high-voltage side of the transformer, n₁ is a number of        windings at the low-voltage side of the transformer, N_(TC) is        the tap position of the tap changer, Δn is a change in the        number of windings at the high-voltage side for a given tap        position, U_(LV) defines the actual voltage at the low-voltage        side of the transformer, I_(reactive) is a reactive current of        the transformer, and X_(HV) is an impedance of the transformer;    -   wherein an over-voltage condition is detected if the estimated        voltage U_(HV-estimate) is determined to be within an        over-voltage range.

According to a second aspect of the invention there is provided acontroller for controlling a power generating system comprising a powergenerator, a generator side converter, a grid side converter, atransformer, a tap changer for a wind turbine, the controller beingarranged to be connected to the power generating system and configuredto control the power generating system according to the method of anypreceding claim.

According to a third aspect of the invention there is provided a powergenerating system for a wind turbine which is connected to an externalelectrical grid, the power generating system comprising a converter, atransformer, a tap changer, and a controller, the controller comprising;an input arranged to receive an input signal indicative of a voltage ofthe electrical grid; a determining module arranged to determine a demandfor an increase in output voltage from the power generating system basedon the input signal; a transformer control module arranged to determinea transformer control signal to operate the tap changer to tap-down thetransformer; a converter control module arranged to determine aconverter control signal to operate the converter; and an outputarranged to transmit the converter and transformer control signals tothe power generating system; wherein the determining module isconfigured to determine a partial-load condition of the converter whichcorresponds to the converter being configured to output a voltage whichis substantially below its maximum output voltage, and upon determiningthe partial-load condition, the transformer control module is configuredto operate the tap changer to tap-down the transformer, and theconverter control module is configured to operate the converter toprovide the required increase in output voltage from the powergenerating system.

It will be appreciated that the foregoing represents only some of thepossibilities with respect to a control method for controlling a powergenerating system. Accordingly, it will be further appreciated thatembodiments of a control method which include other or additional methodsteps remain within the scope of the present invention. Additionalsub-method steps may relate to other method steps relating to theoperation of a wind turbine.

The set of instructions (or method steps) described above may beembedded in a computer-readable storage medium (e.g. a non-transitorystorage medium) that may comprise any mechanism for storing informationin a form readable by a machine or electronic processors/computationaldevice, including, without limitation: a magnetic storage medium (e.g.floppy diskette); optical storage medium (e.g. CD-ROM); magneto opticalstorage medium; read only memory (ROM); random access memory (RAM);erasable programmable memory (e.g. EPROM ad EEPROM); flash memory; orelectrical or other types of medium for storing suchinformation/instructions.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 illustrates a wind turbine including nacelle and a plurality ofrotor blades;

FIG. 2 illustrates a power generating system of the wind turbine of FIG.1;

FIG. 3 illustrates an alternative power generating system of the windturbine of FIG. 1;

FIG. 4 shows a set of International Electrotechnical Commission (IEC)wind distribution charts corresponding to four different wind conditionsexperienced by the wind turbine of FIG. 1;

FIG. 5 is a table indicating the distribution of time that the windturbine of FIG. 1 is subjected to different wind conditions;

FIG. 6 illustrates a controller of the power generating system of FIG.2;

FIG. 7 is a visual representation of the determined configurationparameters of the power generating system of FIG. 2, as determined bythe controller of FIG. 6;

FIG. 8 is an optimised PQ chart corresponding to the power generatingsystem of FIG. 2;

FIG. 9 shows a flow chart illustrating a first method of controlling thepower generating system of FIGS. 2 and 3;

FIG. 10 shows a flow chart illustrating a second method of controllingthe power generating system of FIGS. 2 and 3;

FIG. 11 is a table describing a number of strategies for mitigating anextreme over-modulation condition of the power generating system ofFIGS. 2 and 3;

FIG. 12 shows a flow chart illustrating a third method of controllingthe power generating system of FIGS. 2 and 3;

FIG. 13 is a schematic representation of a droop control strategy forthe power generating system of FIGS. 2 and 3; and

FIG. 14 is a schematic of a tap changer of the power generating systemof FIGS. 2 and 3.

DETAILED DESCRIPTION

FIG. 1 shows a wind turbine 10 including a tower 12, a nacelle 14rotatably coupled to the top of the tower 12, a rotor 16 including arotor hub mounted to the nacelle 14, and a plurality of wind turbinerotor blades 18—in the described example, three rotor blades—which arecoupled to the rotor hub. The nacelle 14 and rotor blades 18 are turnedand directed into the wind direction by a yaw system. The wind turbine10 is shown in its fully-installed form suitable for operation; inparticular, the rotor 16 is mounted on the nacelle 14 and each of theblades 18 are mounted on the rotor hub. The rotor 16 is rotatable byaction of the wind during operation of the wind turbine 10 in order toconvert the kinetic energy of the wind into rotational energy of therotor 16.

With particular reference to FIGS. 2 and 3, the nacelle 14 houses apower generating system 20, 120 which is capable of converting therotational energy of the rotor 16 into electric power which can besupplied to an electrical grid 28—or grid 28. Note that FIGS. 2 and 3illustrate two different versions of power generating architectureswithin which embodiments of the invention may apply, so both generatingsystem will be discussed here in broad terms so as to introduce the mainfunctional components. During operation of the wind turbine 10, the windinduced rotational energy of the rotor 16 is transferred via a shaft 22to a generator 24, 124. A first portion 22 a of the shaft 22 isrotatably coupled at one end to the rotor 16 and at another end to agearbox assembly 26. The gearbox assembly 26 is arranged to transfer therotational speed of the first shaft portion 22 a to the generator 24 viaa second shaft portion 22 b which is rotatably coupled therebetween. Thegearbox assembly 26 includes a step-up ratio which increases therotational speed of the second shaft portion 22 b compared to the firstshaft portion 22 a. In this way, the gearbox assembly 26 steps up theinherently low rotational speed of the rotor 16 for the generator 24 toefficiently convert the rotational mechanical energy to electricalenergy, which is fed into the grid 28 via at least one electricalconnection.

Each of the exemplary power generating systems 20, 120 described hereinare provided with a gearbox assembly 26 arranged to transfer powerbetween the rotor 16 and the generator 24. However, the methods andsystems according to the present invention would also apply to a powergenerating system comprising a direct-drive connection between the rotorand the generator, as would be readily understood by a person havingordinary skill in the art.

The generator 24 of the power generating system 20 shown in FIG. 2 is athree-phase, double-fed induction (asynchronous) generator (DFIG), whichincludes a generator stator which is magnetically coupled to a generatorrotor coil (not shown). A portion of the electrical power generated bythe generator 24 is transferred from the generator stator to a converterassembly 30 of the power generating system 20, and the remainder of thegenerated power is transferred from the generator rotor coil directly tothe grid 28.

According to an alternative arrangement of the power generating system120, the generator 24 is a permanent magnet generator 124, asillustrated in FIG. 3. The permanent magnet generator 124 includes apermanent magnet rotor (not shown) from which substantially all of thepower generated from the generator 124 is directed to grid 28 via aconverter assembly 30.

In each of the power generating systems 20, 120 shown in FIGS. 2 and 3,the respective generators 24, are electrically connected to a generatorside converter 32—or a machine side converter—of the converter assembly30. The generator side converter 32 is connected to a grid sideconverter 34 via a DC-link 38 which includes a positive rail, a negativerail and one or more DC-link capacitors coupled therebetween. Duringoperation of the wind turbine 10, the DC-link capacitors are charged bythe DC output current from the generator side converter 32, whichthereby supplies DC power to the grid side converter 34. An output ACcurrent from the grid side converter 34 is supplied via a transformer 36to the grid 28.

The transformer 36 of each of the power generating systems 20, 120includes a tap changer assembly 40—or tap changer 40, which enablesswitching, or ‘tapping’, between different turn ratios of thetransformer 36 in order to control the voltage output to the grid 28.The power generating system 20, 120 includes a controller 42 arranged tomonitor and control the operation of the power generating system 20,120, as will be described in more detail below.

With reference to FIGS. 4 and 5, the operation of the power generatingsystem 20, 120 will now be described with reference to the differentoperational conditions of the wind turbine 10. In particular, FIG. 4shows a set of International Electrotechnical Commission (IEC) winddistribution charts corresponding to four different wind conditions I,II, III and IV. Each of the wind distribution charts illustrates atypical distribution vs. frequency of the wind speeds associated witheach of the four wind condition categories I-IV.

FIG. 5 is a table summarising the data shown in FIG. 4, and relates to awind turbine 10 which is configured to deliver a maximum power output of6.16 MW. The data in FIG. 5 represents an example of many possible

In particular, the converter assembly 30 is rated to deliver a maximumactive power P of 5.6 MW when the tap changer 40 is set to a neutralcondition. Tapping up the tap changer 40 by +10% increases the maximumoutput power to 6.16 MW. Conversely, by tapping down the tap changer 40by −10%, the minimum active power P is reduced to 4.98 MW. Tapping downthe tap changer 40 also increases the current by 11%.

The columns labelled W, X, Y and Z represent four different operatingconditions of the power generating system 20, 120. Each of the fouroperating conditions W, X, Y and Z is associated with a different rangeof wind speeds experienced by the wind turbine 10. For example,operating condition W is associated with a wind speed of 0 to 10 metresper second (m/s), which corresponds to an operating power output fromthe power generating system 20, 120 of between 0 to 4.98 MW.

Operating condition W corresponds to an under-modulation of the windturbine 10 in which the wind speed is below an optimum level for powergeneration, whereas operating condition X corresponds to an optimum windcondition for power generation. For certain situations, the operatingconditions Y and Z correspond to an over-modulation condition and acut-out condition, respectively. The over-modulation conditioncorresponds to the situation in which the wind turbine 10 is required tooperate above its optimum operating condition but still withinmanageable range of the power generating system 20, 120. The cut-outcondition corresponds to a situation in which the wind speed is so highthat the power generating system 20, 120 is shut down in order toprotect the components of the wind turbine 10.

Each row of the table describes the relative time in which the powergenerating system 20, 120 is operated according to each of the fouroperating conditions 1-4, as a percentage of the total wind turbineoperation. The four rows correspond to the different wind conditioncategories I-IV. For example, the power generating system 20, 120 isoperated according to operating condition W for 63.21% of the timeduring wind conditions corresponding to wind category IV. Moregenerally, operating condition W occurs between 63-93% of the time,scheme X occurs between 3-7% of the time and scheme Y occurs between3.5-28% of the time. By contrast, scheme Z has a significantly rarerincidence rate, occurring less than 1.8% of time.

The power generating system 20, 120 is controlled according to adifferent control method—or operating scheme—depending on which of theoperating conditions W, X, Y and Z the wind turbine 10 is operatingwithin. Each control method is implemented by a controller 42 of thepower generating system 20, 120 according to the present invention.

The controller 42 includes a number of processing modules 50, 52, 54including a determining module 50, a converter control module 52 and atransformer control module 54, as shown in FIG. 6. The controller 42also includes an input 46 and an output 48 which are arranged tofacilitate communications between the controller 42 and the powergenerating system 20, 120, as illustrated by the connections shown inFIG. 2 between the controller 42, the transformer 36 and the converterassembly 30.

The primary function of the controller 42 is to operate the powergenerating system 20, 120 according to one or more control schemes, aswill described in more detail below. Each of the modules 50, 52, 54 isconfigured to perform a variety of functionalities depending on whichcontrol scheme is being implemented by the controller 42. Generally, thedetermining module 50 is arranged to monitor and interpret differentparameters, including environmental and operational parameters of thewind turbine 10, which may affect the power generation system 20, 120.The converter control module 52—or converter module 52—is configured tocontrol the operation of the converter assembly 30. Similarly, thetransformer control module 54—or transformer module 54—is arranged tocontrol the transformer 36 via the tap changer 40.

The input 46 and output 48 each include one or more signal converterswhich are arranged to permit signals transmitted to and from thecontroller 42. For example, the input signal converter is configured toconvert the signals received from the components of the power generatingsystem 20, 120 into a medium that can be interpreted by the controller42. Similarly, the output signal converter is arranged to convertoutputted signals into a medium that can be understood by the componentof the power generating system 20, 120.

The input 46 is arranged to receive a variety of inputs—or inputsignals—relating to both the configuration and operation of the powergenerating system 20, 120. A configuration input—or configuration inputsignal—defines how a particular component of the power generating system20, 120 has been configured so as to carry out its function. Anexemplary configuration input includes information relating to the rangeof voltages which the converter assembly 30 is operable to output duringnormal use. Alternatively, the configuration input includes a number oftap change positions of the tap changer 40. The different tap changerpositions are, in turn, indicative of a corresponding turn ratio of thetransformer 36.

The configuration input signals are transmitted to the controller 42 bya user of the power generating system 20, 120 via a human machineinterface (not shown). Alternatively, or in addition, each of theconfiguration inputs may be provided in the form of a lookup table whichis stored on a suitable storage medium.

The input 46 is also arranged to receive operating inputs—or operatinginput signals—which define an operating condition of a particularcomponent during operation of the power generating system 20, 120. Anexemplary operating input includes voltage and/or electric currentmeasurements received from one or more voltage and electric currentsensors (not shown) of the power generating system 20, 120. The voltageand current sensors are electrically connected to the components, or toelectrical connections between the components, of the power generatingsystems 20, 120, as would be readily understood by the skilled person.An alternative operating input may be indicative of the current tapchange position of the tap changer 40. Further exemplary operatinginputs relate to the operating parameters of the converter assembly 30,including the detectable operating parameters of the machine sideconverter 32, the grid side converter 34 and the DC-link 38.

Under-Modulation Control Scheme 60

An exemplary control method 60 for operating the power generating system20, 120 according to a first aspect of the present invention will now bedescribed with reference to FIGS. 7, 8 and 9. The control method 60 isparticularly suited for controlling the power generating system 20, 120in situations when the wind turbine 10 is operating under schemes W andX.

The control method 60 corresponds to a wind turbine 10 which is operatedto support the voltage and power levels in the grid 28. Suchgrid-feeding, or grid-supporting, configurations require the powergenerating system 20, 120 to deliver active and reactive power P, Q tothe grid 28. In this situation, the controller 42 acts as a primarycontrol by setting an operation point for each of the converter assembly30 and tap changer 40, as a function of their respective capacities, inorder to maximise the power generating capacity of the wind turbine 10.Operation of the power generating system 20, 120 may be controlled by asecondary control (i.e. an electrical grid controller) which is arrangedto set active and reactive power references P*, Q*. The main objectiveof the secondary control is to minimise the voltage and the frequencydeviations within the grid 28.

The determining module 50 is arranged to determine a set of controlparameters based on the configuration of the components of the powergenerating system 20, 120. The converter and transformer modules 52, 54are then arranged to calculate, in dependence on receiving a demandsignal, one or more control signal(s) for controlling the powergenerating system 20, 120.

The determining module 50 is primarily arranged to determine the set ofcontrol parameters when the power generating system 20, 120 is in anoffline condition, i.e. when the wind turbine 10 is not supplying powerto the grid 28. Further, the determining module 50 may determine thecontrol parameters prior to the power generating system 20, 120 beinginstalled within the wind turbine 10. In addition, the determiningmodule 50 can be operated to update the control parameters in responseto a change in the configuration of the system's components. Suchchanges may occur following the replacement of one or more of thecomponents of the power generating system 20, 120. In contrast to thedetermining module 50, the converter and transformer modules 52, 54 areconfigured to be operated during an online condition of the powergenerating system 20, 120, i.e. during the operation of the wind turbine10.

The operation of the determining module 50 will now be described withparticular reference to FIG. 7, which shows a schematic of the controlparameters as determined by the determining module 50. The determiningmodule 50 receives a configuration input relating to the converterassembly 30 and transformer 36. The determining module 50 is arranged todetermine, from the configuration input corresponding to the converterassembly 30, that the converter assembly 30 is configured to operate inseven distinct operating modes, labelled C1, C2, C3, C4, C5, C6 and C7in FIG. 7.

Each of the converter operating modes C1-C7 correspond to a converteroutput voltage U_(C) which range from 0.87 per unit system (pu), for C1,to 1.13 pu, for C7. In particular, converter operating modes C1-C7correspond to converter output voltages U_(C) of 0.87 pu, 0.9 pu, 0.95pu, 1.0 pu, 1.05 pu, 1.1 pu and 1.13 pu, respectively. It will beappreciated that the seven voltage modes C1-C7 are described herein asexemplary operating modes of the converter assembly 30. For example, theconverter assembly 30 may be configured to output a range of converteroperating voltages which are separated by 0.1 pu, or alternatively lessthan 0.1 pu. In some situations, the operation of the converter may bedefined by a greater or smaller number of operating modes, as would bereadily understood by the skilled person.

The tap changer 40 is configured to apply a tap change adjustment—or tapchange value—to each of the operating modes C1-C7 of the converterassembly 30. The tap change adjustment defines the extent to which thetransformer 36 is tapped up or down by the tap changer 40. For example,the determining module 50 determines that the tap changer 40 should beconfigured to apply a tap change adjustment of plus or minus ten percent(+/−10%) to the converter output voltage U_(C) which it receives fromthe converter assembly 30. It will be appreciated by the skilled personthat the tap change adjustment values described above (i.e. +/−10%represent an exemplary aspect of the determining module 50. The tapchanger may be configured to apply a range of adjustments to thetransformer, depending on the requirements of the power generatingsystem.

For the exemplary case of operating mode C1, the tap changer 40 iscapable of tapping-up and tapping-down the converter output voltageU_(C) (i.e. 0.87 pu) such that the ‘tap change adjusted’ output voltagerange is between 0.77 pu and 0.97 pu. Similarly, the nominal range ofoutput voltages corresponding to operating mode C2 is between 0.7 pu and0.9 pu.

The determining module 50 is arranged to determine the nominal range ofoutput voltages obtained by tapping up and tapping down the converteroutput voltages U_(C) which correspond to the converter operating modesC1-C7. Put another way, the determining module 50 is configured todetermine the tap change adjusted output voltage range for each of theconverter operating modes C1-C7. The determined operating parameter data(i.e. the range of ‘tap change adjusted’ converter output voltages) isstored on a memory device 55 of the controller 42.

Owing to the available tap change adjustment, it will be appreciatedthat there is significant overlap between the different ranges of tapchange adjusted output voltage, as represented by the horizontal barsshown in FIG. 7. For example, the non-adjusted converter output voltageU_(C) corresponding to operating mode C1 is 0.87 pu. A substantiallysimilar output voltage can also be achieved by switching the converterassembly 30 to converter operating mode C2 (i.e. with a converter outputvoltage of 0.90 pu) and by tapping down the converter output voltage by−5%. Alternatively, the same output voltage can also be achieved bytapping down the output voltage of converter operating mode C3 (i.e.0.95 pu) by 10%.

Each of these three configurations result in a different level ofreactive power Q being available to be transmitted into, or absorbedfrom, the grid 28, despite producing substantially the same outputvoltage. By operating the converter assembly 30 according to the firstconfiguration (i.e. applying converter operating mode C1 with noadjustment to the tap changer 40) it results in a reactive power value Qof 1.018 pu, for example. By comparison, the reactive power value Qcorresponding to the second configuration (i.e. converter operating modeC2 combined with a −5% tap change adjustment) is 1.341 pu and for thethird configuration (i.e. converter operating mode C3 combined with a−10% tap change adjustment) the reactive power value Q is 1.396 pu.

Accordingly, the determining module 50 is arranged to determine thereactive power value Q corresponding to each possible configuration ofthe converter assembly 30 and tap changer 40. The determining module 50is also configured to determine the converter assembly 30 and tapchanger 40 operating parameters corresponding to each of the respectivereactive power values. The operating parameters are then stored, alongwith the tap change adjusted voltage data, in the memory device 55 ofthe controller 42.

Having calculated the associated reactive power value for each of thepossible operating parameter combinations, the determining module 50 isfurther configured to determine a set of optimised operating parametersS1, S2, S3, S4, S5, S6 and S7, by which the system can be controlled inorder to deliver a desired output to the grid 28. Each of the operatingparameters S1-S7 is determined to control the system to transmit arequired voltage output to the grid. To achieve this, each operatingparameter S1-S7 comprises a converter output voltage (i.e. correspondingto one of the converter operating modes C1-C7) and a tap changeradjustment (i.e. applying an adjustment to the transformer 36 of between+10% and −10%, for example).

The optimised operating parameters S1-S7 are further configured tomaximise the power generating capability of the wind turbine generator10. According to an exemplary arrangement of the controller 42, thedetermining module 50 is configured to determine the converter assembly30 and the tap changer 40 operating parameters which maximise thereactive power Q output of the power generating system 20, 120. Forexample, the first optimised operating parameter S1 comprises operatingthe converter assembly 30 in operating mode C3 and with the tap changer40 being configured to tap down the transformer 36 by −10%. As describedabove, this combination of converter assembly 30 and tap changer 40operating parameters provides the maximum reactive power output of 1.396pu for the corresponding output voltage of 0.87 pu.

By tapping down the tap changer 40 when in a partial load condition, theoptimised operating parameter S1 advantageously increases the overvoltage ride through (OVRT) capabilities of the system 20, 120 comparedto a system which does not include a tap changer. This is because thewind turbine generator 10 only operates at full load for a portion ofits operational life, and the rest of the time it will operate in apartial load condition. Accordingly, by tapping down the converteroutput voltage during partial load conditions, and thereby increasingthe converter operating current, the associated converter losses arereduced for the majority of the converter's operational life.

As a further example of the operation of the determining module 50according to the present invention, we now refer to converter operatingmode C7 in FIG. 7 for which the nominal converter output voltage is 1.13pu. The optimised operating parameter S7 corresponding to this outputvoltage includes operating the converter assembly 30 according tooperating mode C6 and then tapping up the tap changer 40 by 5%.Tapping-up the voltage +5% increases the reactive power output Q by 5%with the same current passing through the converter assembly 36. Thus,the power generating system 20, 120 is able to output more reactivepower Q than an equivalent power generating system which does notinclude a tap changer.

Furthermore, by applying the optimised control protocol as describedabove, the power generating system 20, 120 is able to increase the upperlimit of its ‘normal’ operating range from 1.13 pu to 1.23 pu.Furthermore, the over-modulation range of the power generating system20, 120 is also increased so that it extends between 1.23 pu and 1.44pu, owing to the influence of the tap changer control as describedherein. By contrast, an equivalent power generating system which doesnot include a tap changer 40 would have an upper limit of its ‘normal’operating range of 1.13 pu.

The optimised operating parameters S1-S7 are stored on the memory device55 as a control protocol, which can be read by the converter andtransformer modules 52, 54 during operation of the power generatingsystem 20, 120, as will be described later. The optimised operatingparameter data can then be updated by the determining module 50 inresponse to receiving updated configuration inputs.

The control protocol can be presented as a series of active power vs.reactive power PQ charts, as shown in FIG. 8. The PQ charts representthe power generating capability of the power generating system 20, 120when it is controlled with respect to the optimised operating parametersS1-S7. Each PQ chart indicates the reactive power Q transmitted to thegrid 28 with respect to the active power P. The different shaded linescorrespond to the power generating capability of the power generatingsystem 20, 120, when it is operated according to the respectiveoperating parameters S1-S7.

The highlighted regions A, B, C and D each represent a point on the PQchart which is of particular interest to the operation of the powergenerating system 20, 120. For example, region A corresponds to asituation in which the power generating system 20, 120 may be controlledto transmit reactive power +Q into the grid 28, whereas region Ccorresponds to a situation in which the system is controlled to absorbreactive power −Q from the grid.

The above described determination of the optimised operating parametersS1-S7 (as shown in FIG. 7) corresponds to point A on the PQ charts ofFIG. 8. The determining module 50 is also configured to determine theoptimised operating parameters for each of the points B, C and D. Theresulting sets of optimised operating parameters are used to providemaximised PQ charts for the power generating system 20, 120, whichthereby define the maximum power generating capability of the windturbine 10.

As described above, the control protocol comprises a set of operatingparameters which result in an optimised power output from the system 20,120. In this situation, the desired change in reactive power output Qmay be achieved without changing the operating mode C1-C7 of theconverter assembly 30. With particular reference to FIG. 7, each of theoptimised operating parameters S4-S7 involves operating the converter inoperating mode C6. Accordingly, the desired power output voltage isobtained by switching the tap changer 40 between −10% and +5%. Thus, thetap changer 40 is used to provide at least a portion of the requiredincrease in reactive power output Q. Advantageously, this controlstrategy reduces the number of times that the converter assembly 30 mustbe adjusted in order to change the reactive power output of the system,which thereby increases the operational life of the converter assembly30.

With particular reference to FIG. 6, the controller 42 is configured tocontrol the power generating system 12, 120 based on the optimisedoperating parameters S1-S7. In particular, the input 46 is arranged toreceive a demand input—or demand input signal—which is indicative of arequirement to change the output voltage U_(O) of the power generatingsystem 20, 120. Such a demand signal may include, for example, a requestfor the power generating system 20, 120 to support the voltage in thegrid 28.

The converter module 52 is configured to determine a converter controlsignal for controlling the converter assembly 30 and transformer module54 is configured to determine a transformer control signal to controlthe tap changer 40 in order to meet the required change in outputvoltage and/or reactive power Q. According to an exemplary arrangementof the controller 42, the control signals are determined so as toconfigure the converter assembly 30 and the tap changer 40 based on theoptimised operating parameters S1-S7, as defined in the controlprotocol.

For example, upon receiving an input signal indicative of a demand forthe power management system 20, 120 to deliver an output voltagecorresponding to 0.87 pu, the determining module 50 determines that theoptimised operating parameter S1 is capable of achieving the requiredoutput. The converter module 52 then determines a converter controlsignal to control the converter assembly 30 to output a voltage of 0.9pu (i.e. to operate the converter assembly 30 according to operatingmode C3). The transformer module 54 also determines a tap changercontrol signal to control the tap changer 40 to tap down the convertervoltage by a determined tap change adjustment value, such as −10%.

According to this arrangement, the controller 42 is configured tooperate the power generating system 20, 120 according to thepre-determined control protocol, as previously determined by thedetermining module 50. The output 48 is arranged to transmit the controlsignals to the relevant components of the power generating system 20,120, in order to control the operation of the converter assembly 30 andthe tap changer 40, as necessary.

According to an alternative arrangement of the controller 42, theprocessing modules 50, 52, 54 are again configured to control the powergenerating system 20, 120 in dependence on receiving a demand for anadjustment in the output voltage U_(O) to the grid 28. However, in thiscase the converter and transformer modules 52, 54 are arranged tooperate the converter assembly 30 and the tap changer 40, respectively,without the need to consult the pre-determined operating protocol. Inthis way, the controller 42 is arranged to determine one or more controlsignals to control the power generating system 20, 120 according to adynamic control scheme 60—or control method 60.

An exemplary method of operating the power generating system 20, 120according to the dynamic control scheme 60 will now be described withreference to FIG. 9. The control method 60 commences with a first step62 in which the controller 42 receives, via the input 46, an inputsignal indicative of a demand for an adjustment in the output voltageU_(O) of the power generating system 20, 120. The demand for an increasein output voltage U_(O) may correspond to a demand for increased poweroutput PQ from the power generating system 20, 120. Accordingly, thecontroller 42 is configured to monitor the input signal to detect a gridvoltage U_(G), which requires an adjustment in the output voltage U_(O).

In a second method step 64, the determining module 50 determines apartial-load condition of the converter assembly 30. The partial-loadcondition corresponds to the converter assembly 30 being configured tooutput a converter output voltage U_(C) which is substantially below themaximum converter output voltage. The partial-load condition also refersto situations wherein the power generating system 20, 120 is operatedbelow a maximum power output for which the system is rated. In thissituation, the partial-load condition refers to when the wind turbine 10is operated during operating conditions W and/or X, i.e. below the‘over-modulation’ operating condition Y.

It is noted that in each of the exemplary control methods describedherein, the turbine load (i.e. the maximum mechanical load that theturbine can accommodate) is not tied to a specific range of modulationindex values. Accordingly, the power generating system may not belimited to operating according to prescribed modulation range duringcertain turbine load conditions.

According to an exemplary operation of the wind turbine 10, the powergenerating system 20, 120 is initially controlled to operate at fullactive power P with an output voltage U_(O) of 0.95 pu, To achieve this,the converter assembly 30 is operated in converter operating mode C3,and the tap changer 40 is configured in a neutral position. Thiscombination of operating parameters generates a reactive power output Qof 1.396 pu, which corresponds to region A of the PQ chart shown in FIG.8.

The controller 42 then receives an input signal indicative of a demandto increase the reactive power output Q to 1.451 pu. Upon receiving theinput signal, the determining module 50 determines a number of possibleconfigurations of the converter assembly 30 and tap changer 40 whichwould be able to achieve the required reactive power output Q. A firstconfiguration comprises switching the converter assembly 30 to operatein converter mode C4 (corresponding to a converter output voltage U_(C)of 1.05 pu) and retaining the tap changer 40 in its neutral position.Alternatively, the same reactive power output Q could also be achievedby switching the converter assembly 30 to operate in converter mode C4and then tapping down the transformer 36 by 5% or 10%.

In a third method step 66, the converter module 52 determines a controlsignal to control the converter assembly 30 to operate in converter modeC4 and the transformer module 54 determines a control signal to tap downthe transformer 36 by either 5% or 10%. Thus, each control signal isconfigured to operate the tap changer 40 and the converter assembly 30in order to provide the required adjustment in reactive power outputfrom the power generating system 20, 120.

By tapping down the tap changer 40 during the partial-load configurationof the converter assembly 30, the power generating system 20, 120 isable to achieve the requested reactive power output Q whilst minimisingits output voltage U_(O). The advantageous method also increases thepotential over-voltage ride-through response during higher loadconditions, thereby increasing the power generating system's 20, 120ability to respond to demands from the grid 28. Put another way, thecontroller 42 retains its ability to adjust the tap changer 40 inresponse to future fluctuations in the grid voltage.

According to an aspect of the present invention, the control method 60comprises controlling the tap changer 40 to adjust the transformer 36according to a reduced tap changer adjustment, In this way, the tapchanger 40 is controlled to adjust the transformer 36 by a tap changeadjustment which is less than the maximum available tap changeadjustment. For example, the control method 60 may comprise only tappingdown the transformer by 5% instead of tapping down by the maximum 10%.Advantageously, the resulting tap change will still achieve the requiredincrease in reactive power output Q whilst also limiting the number oftap changer adjustments—or tappings, which will thereby prolong the lifeof the tap changer 40. According to an alternative exemplary controlmethod, the tap changer 40 may be controlled to adjust the transformer36 by one or more suitable tap change adjustments being less than themaximum tap change adjustment, as would be appreciated by the skilledperson.

In a final method step 68, the output 48 transmits the control signal(s)to the converter assembly 30 and to the tap changer 40 in order tocontrol the operation of the power generating system 20, 120 to meet thedemand from the grid 28.

As described above, the control method 60 is particularly suited forcontrolling the power generating system 20, 120 in situations when thewind turbine 10 is operating under conditions associated with operatingconditions W and X. Referring again to FIGS. 4 and 5, the operatingconditions W and X correspond to an under-modulation condition of thewind turbine 10. Accordingly, the controller 42 is arranged to receive awind speed input—or wind speed signal—indicative of a wind speed in thearea of the wind turbine 10. The wind speed signal comprises informationrelating to the wind speed in the local vicinity of the wind turbine 10.As such, the input 46 of the controller 42 is configured to receive awind speed signal from a wind speed sensor which is arranged either onor near to the wind turbine 10. Alternatively, the wind speed signal maybe derived from a measure of the torque which is applied by the wind tothe rotor blades 18, as would be readily understood by the person havingordinary skill in the art.

The determining module 52 is configured to determine the speed of thewind that is incident upon the wind turbine 10, based on the wind speedsignal. In turn, the determining module 50 is able to determine whetherthe power generating system 20, 120 is operating in one of the operatingconditions W, X, Y and Z. Upon determining that the wind speed fallswithin at least one of the wind speed ranges associated with W and X(i.e. within an exemplary wind speed range of 0-10 m/s or 10-11 m/s,respectively), the controller 42 then proceeds to control the powergenerating system 20, 120 according to the control method 60, asdescribed above.

Advantageously, the control method 60 increases the capacity of thepower generating system 20, 120 to provide an overvoltage to theelectrical grid 28. However, if an overvoltage capability is notdesired, then the control method 60 may be disabled. In such asituation, the operation of the power generating system 20, 120 mayrevert back to being determined by the pre-determined operatingprotocol, as described above.

If the determining module 50 detects that the wind speed is outside thewind speed range of operating conditions W and X, then the controller 42inhibits control of the power generating system 20, 120 according tocontrol method 60. For example, if the wind speed is determined to be inthe over-modulation range (i.e. operating condition Y), then thecontroller 42 will operate the power generating system 20, 120 accordingto an over-modulation control strategy 160, which is described in moredetail below. Alternatively, if the wind speed is determined to begreater than a determined wind speed threshold value, such as 20 m/s forexample (i.e. falling within the operating condition Z), then thecontroller 42 is configured to initiate a shut-down control strategy inorder to protect the components of the power generating system 20, 120.

The controller 42 is also configured to receive signals indicative ofthe current wind speed so that the determining module 50 can determinewhere the current wind speed sits within wind speed range of therelevant operating condition W, X, Y, and Z.

If the wind speed is determined to be within the range of operatingcondition W (i.e. when the wind speed is between 0-10 m/s), then thecontroller 42 is configured to moderate the extent to which thetransformer 36 is tapped down by the tap changer 40. To achieve this,the controller 42 is configured to only allow tapping down of thetransformer 36 when an operating current I_(C) of the converter assembly30 is below an operating current threshold value, such as 1.0 pu, i.e.when the output power is between 0 to 5.04 MW, for example. Accordingly,the tapping down of the transformer 36 is inhibited when the operatingcurrent I_(C) of the converter assembly 30 is at or above a thresholdcurrent I_(T). The threshold current I_(T) defines a converter operatingcurrent which, if exceeded, may cause damage to the converter assembly30 and/or other components of the power generating system 20, 120.

By moderating the tapping down of the transformer 36 in this way, thecontroller 42 is configured to safely increase the overvoltagecapabilities of the power generating system 20, 120 without causingdamage system 20, 120 or the grid 28. This control strategy also reducesharmonic distortion during overvoltage operating conditions. Accordingto an alternative arrangement of the power generating system, theconverter assembly 30 is configured such that it operates below 1.0 puwhen the output power of the converter assembly 30 is determined to bewithin a range of power values, for example, between 0 and 4.96 MW.

If the wind speed is determined to be within the range of operatingcondition X (i.e. when the wind speed is between 10-11 m/s), thecontroller 42 is further determined to moderate the tapping down of thetransformer 36. To achieve this, the determining module 50 determines atap-down threshold value below which the tap changer 40 cannot be set.The tap-down threshold is determined such that it reduces linearly asthe power output of the wind turbine 10 increases from 5.04 MW to 5.6MW. Accordingly, a maximum tap-down threshold of −10% is determined whenthe wind speed is at 10 m/s, and a minimum tap-down threshold of 0% isdetermined when the wind speed is at 10 m/s. This prevents the operatingcurrent I_(C) of the converter assembly 30 from exceeding a thresholdcurrent I_(T). According to an alternative arrangement of the controller42, the tap down threshold may be determined based on the operatingcurrent I_(C) of the converter assembly 30.

With reference to the control method 60, as described above, thedetermining module 50 is configured to determine a tap change adjustmentvalue which defines the extent to which the transformer 36 is tapped upor tapped down by the tap changer 40. For example, the determiningmodule 50 is configured to determine whether to tap down the transformer36 by either 5% or 10%. In this situation, the determining module 50receives an indication that the wind speed is at 11 m/s. Alternatively,the determining module 50 may detect that the power generating system20, 120 is outputting power at 5.6 MW (i.e. the upper limit of operatingcondition X). Accordingly, the tap-down threshold is set to 0%, therebypreventing any tapping down of the transformer 36. The transformermodule 54 is then configured to generate a control signal which retainsthe tap changer 40 in a neutral position.

It is noted that the controller 42 is configured to allow tapping up ofthe transformer 36 at any time whist the power generating system 20, 120is operated according to operating conditions W or X, since this wouldnot cause any adverse effects to either the grid 28 or the powergenerating system 20, 120.

Over-Modulation Control Scheme 160

According to a second aspect of the invention, the controller 42 isconfigured to control the tap changer 40 and the converter assembly 30according to an over-modulation control scheme 160, as shown in FIG. 10.The control scheme 160 is particularly suited to situations when thewind turbine 10 is operating in wind speeds of between 11-20 m/s (i.e.corresponding to operating condition Y).

The primary function of the controller 42, when operating according tothe control scheme 160, is to increase the operating capabilities of thepower generating system 20, 120 during over-modulation operatingconditions.

In a first method step 162 of the over-modulation control scheme 160—orcontrol method 160—the input 46 is configured to receive a signalindicative of the grid voltage U_(G) of the electrical grid 28. In asecond step 164, the determining module 50 is configured to detect anover-voltage condition requiring a reduction in the output voltage U_(O)of the power generating system 20, 120.

In a third method step 166, the converter module 52 is arranged todetermine a converter control signal to operate the converter assembly30 in order to provide at least part of the required voltage reduction.The transformer module 54 is also arranged to determine a transformercontrol signal to adjust the tap changer 40 to tap down the transformer36 in order provide at least part of the required voltage reduction.

The output 48 is arranged, upon detection of the over-voltage condition,to transmit the converter and transformer control signals in a fourthmethod step 168 to provide the required reduction in the output voltageof the power generating system 20, 120. The converter response mode andthe transformer response mode are initiated so that they are implementedat least partially during the same period of time.

The converter module 52 is configured to operate in a converter responsemode such that it is arranged to control the converter assembly 36 torespond to the over-modulation condition. Similarly, the transformermodule 54 is configured to operate in a transformer response mode suchthat it is able to control the tap changer 40 to adjust the transformer36 to respond to the detected over-modulation condition.

Advantageously, the control method 160 configures the controller 42 toreact to extreme over-modulation voltages in the electrical grid 28 byusing both the converter and transformer response modes to counteractthe over-voltage. The converter assembly 36 can be adjusted quickly toaccommodate the voltage demand. By contrast, the tap changer 40 isslower to react but it provides a more efficient and stable means ofreducing the output voltage U_(O) which can therefore be sustainedindefinitely.

After a period of time following the initiation of the converter andtransformer response modes, the converter module 52 is arranged tocancel the converter response mode. The controller 42 therefore revertsto operating the power generating system 20, 120 according to just thetransformer response mode. The transformer response mode is arranged toretain the tap changer 40 in the required tap changer position so as toprovide the required output voltage U_(O).

According to this exemplary control method, the predetermined period oftime after which the converter response mode is cancelled is determinedby the determining module 50 to be no more than 2 seconds. As such, thepredetermined period provides the controller 42 with sufficient time toinitiate the tap changer 40 to adjust the transformer 36 in accordancewith the transformer response mode control before the converter responsemode is shut down. Alternatively, the predetermined time period may bedetermined in dependence on the severity of the over-modulation, as willbe described in more detail below.

The transformer response mode represents a tap changer adjustment mode86 in which the transformer module 54 is arranged to tap down thevoltage of the transformer in response to the demand from the grid 28.The converter response mode includes a number of different convertercontrol strategies, as shown in the table of FIG. 11. Each of theconverter control strategies can be implemented by the converter module52 in response to the demand for the change in voltage from the grid 28.

A first converter control strategy comprises a DC voltage U_(dc)adjustment mode 82 in which the grid side converter 34 is configured toincrease a voltage across the DC link 38 above a rated value.

A second converter control strategy comprises a reactive powerabsorption mode 84 in which a reduction in voltage is generated across agrid choke of the utility grid 28. Reducing the voltage drop across thegrid choke leads to a reduction in the voltage output requirement forthe converter 34 and therefore a reduced DC voltage output from thepower generating system 20, 120.

A third converter control strategy comprises an over-modulation mode 88in which a modulation index of the grid side converter 90 is increasedto a value in an over-modulation range. The over-modulation range isdetermined by the determining module 50 based on the monitored operatingparameters of the power generating system 20, 120. Alternatively, apre-determined over-modulation range may be stored on in the form of alookup table on the storage device 55. A fourth converter controlstrategy comprises a pulse wave modulation blocking mode 90 in which thegrid side converter 90 is operated to inhibit pulse wave modulation andallow negative power flow through a DC link chopper of the DC link 38.

Upon detecting the over-modulation condition, the determining module 50is arranged to determine the severity of the over-modulation condition.To achieve this, the determining module 50 assigns the detectedover-modulation with a severity value based on the magnitude of therequired change in output voltage U_(O). The converter and transformermodules 52, 54 are then configured to initiate at least one of theconverter and transformer control strategies based on the determinedseverity value. A first severity value corresponds to the grid voltageU_(G) being within a range that requires an output voltage U_(O) whichcan normally be output by the converter assembly 30 (i.e. a modulationindex of less than 1 pu). The first severity value corresponds to a gridvoltage U_(G) which corresponds to a modulation index in a linear rangeof control for the power generating system. A second severity valuecorresponds to the grid voltage U_(G) being within a range that requiresthe converter assembly 30 to generate a voltage that is within anover-modulation voltage range (i.e. a modulation index of between 1 puand 1.1 pu). A third severity value corresponds to the grid voltageU_(G) being such that it requires a voltage that exceeds theover-modulation voltage range (i.e. a modulation index greater than 1.1pu). The third severity value corresponds to a grid voltage U_(G) valuebeing greater than a maximum synthesizable range of the power generatingsystem.

Upon determining that the detected modulation corresponds to the firstseverity value, the transformer module 54 controls the tap changer 40 totap down the transformer 36 according to the tap changer adjustment mode86, and the converter module 52 controls the converter assembly 30 toreduce the voltage generated across the grid choke of the DC link 38 ofthe converter in accordance with the reactive power absorption mode 84.

In the situation where the determining module 50 determines that thedetected modulation corresponds to the second severity value, then thetransformer module 54 is configured to initiate the tap changeradjustment mode 86 and the converter module 52 initiates at least one ofthe reactive power absorption mode 84, the over-modulation mode 88 andthe U_(dc) adjustment mode 82.

Upon determining that the detected over-modulation is within the thirdseverity value range, the transformer module 54 is configured toinitiate the transformer response mode and the converter module 52 isarranged to initiate at least one of the reactive power absorption mode84, the over-modulation mode 88, the U_(dc) adjustment mode 82 and thepulse wave modulation blocking mode 90. Advantageously, the determiningmodule 50 is configured to engage more of the converter controlstrategies when a severe over-modulation condition is detected, in orderto provide a sufficiently large voltage change as required by the grid28.

For each of the scenarios relating to the three different severityvalues, the converter module 52 may be configured to initiate one ormore of the different converter control strategies 82, 84, 88 and 90.Once the determining module 50 has determined which of the severitylevels corresponds to the detected over-modulation condition, thedetermining module 50 is then also configured to determine which of therelevant converter and transformer control strategies should beinitiated. The determining module 50 is also arranged to determine whenthe chosen control strategies should be initiated, and for how long. Forexample, in some instances the converter module 52 may initiate all ofthe relevant control strategies at the same time, and in conjunctionwith the transformer control strategy.

The determining module 50 is also arranged to determine the duration ofeach of the converter and transformer control strategies in order toprovide an optimal response to the required voltage change. To achievethis, the determining module 50 is configured to assign a rank to eachof the transformer and converter control strategies 82, 84, 86, 88 and90. The rankings, as shown in the table of FIG. 11, are determined basedon the speed at which the control strategy can be implemented in orderto counteract the over-modulation condition. The rankings also take intoaccount the stability of the control strategy and their respectivecapacity to affect the over-modulation condition.

The reactive power absorption mode 84 can be applied for relatively longperiods (i.e. a number of minutes) it does involve some degree ofconverter control, which can affect the overall operation of theconverter assembly 30. The U_(dc) adjustment mode 82 should be limitedto a duration of just a few seconds because high DC voltages in theconverter assembly 30 can impact the stability of an insulated-gatebipolar transistor of the power generating system 20, 120. For example,the reliable operation of the transistor can be affected by cosmic rays.

The over-modulation mode 88 involves allowing the converter assembly 30to operate with up to 10% additional voltage range and it can be appliedquickly (i.e. within 1 to 2 seconds) but this approach can causehigh-harmonic output from the converter assembly 30. The pulse wavemodulation blocking mode 90 can be applied quickly but it is mosteffective when only applied for a very short duration (i.e. less than 2seconds) because it can generate a negative power flow which must bedissipated in the DC link chopper in order to prevent the voltage in theDC link 38 from increasing too much. The tap changer adjustment 86 canbe applied indefinitely, and it is highly efficient but it has a slowresponse time because it takes several seconds from the initialoperation of the tap changer 40 to adjust the transformer 36 before theactual reduction in the voltage can occur.

The rankings may be assigned differently to each the severity values,depending on the preferred means of counteracting the voltage demand.The rankings are pre-determined by the determining module 50, accordingto the control method 160 and then stored in the storage device 55 inthe form of a lookup table. Alternatively the rankings may be pre-loadedinto the storage device 55 before the controller 42 is installed withinthe wind turbine 10.

According to the exemplary arrangement of the control method 160, asshown in FIG. 11, the determining module 50 is configured to assign aranking of 1 to the tap changer adjustment mode 86, and a ranking of 2to the reactive power absorption mode 84. Due to its greater ranking thereactive power absorption mode 84 will be initiated in preference to thetap changer adjustment mode 86 because the required voltage level issuch that it can be addressed by the operation of the converter assembly36 within its normal operation. If the over-modulation conditioncontinues at the current level (i.e. severity level 1), despite theapplication of the reactive power absorption mode 84, then the tapchanger adjustment mode 86 is also initiated. The reactive powerabsorption mode 84 may then be cancelled after a pre-determined period,leaving the tap changer adjustment mode 86 to provide the requiredchange in voltage, for an indefinite period, or until theover-modulation severity level changes.

If an over-modulation condition falling within the voltage range of thesecond severity level is detected, then the tap changer adjustment mode86 is required to provide at least some of the required change in outputvoltage U_(O). Each of lower ranked converter control strategies (i.e.the reactive power absorption mode 84, the over-modulation mode 88 andthe U_(dc) adjustment mode 82) are initiated at the same time as the tapchanger adjustment mode 86 in order so that they can start to addressthe over modulation in the period before the tap changer adjustment mode86 can respond.

The converter control strategies are then cancelled after apre-determined period of time according to their assigned ranking. Theover-modulation mode 88 and the U_(dc) adjustment mode 82 are cancelledafter a first determined period of time, for example, less than twoseconds following the initiation of the converter response mode. Thereactive power absorption mode 84 is then cancelled after a seconddetermined period of time, for example one minute. Advantageously, theconverter response mode provides a fast response which helps to mitigatethe initial risk posed by the over-modulation condition. Then, once thetransformer response mode has been implemented, the converter responsemode can then be cancelled in a step-wise fashion, in order to increasethe operational stability of the converter assembly 30.

Droop Control Scheme 260

Each of the first and second control methods 60, 160 correspond to awind turbine 10 which is operated to support the voltage and powerlevels in a grid 28. According to a third aspect of the presentinvention the power generating system 20, 120 may be controlled toprovide enhanced power generating capacity when operating in agrid-forming mode.

Grid-forming power generators are typically arranged to set the voltagethat will be supplied to the loads on the grid. It is known to controlpower generating systems according to a droop control scheme when thewind turbine 10 is configured to operate in a grid-forming mode. Suchcontrol schemes are used, typically, in order to control power sharingwithin electrical power grids, thereby removing the need to provideseparate communication networks to coordinate the operation of the powergenerating systems connected to the electrical grid. However, such droopcontrol strategies can lead to voltage amplitude errors, whichcompromise the output power capability of the power generating system.

According to a third aspect of the present invention the powergenerating system 20, 120 is controlled according to a droop controlscheme 260—or control method 260—in order to extend the power outputcapacity of the power generating system 20, 120 by controlling the tapchanger 40 to overcome the deficiencies of known droop controlstrategies.

The control method 260 will now be described with reference to FIGS. 12and 13. The control method 260 commences with a first step 262 in whichthe determining module 50 is arranged to determine that the powergenerating system 20, 120 is operating in a grid-forming configurationby outputting a reference voltage U_(ref) from the converter assembly 30to the electrical grid 28. The power generating system 20, 120 is alsoconfigured to update the reference voltage U_(ref) in order to maintainthe grid voltage U_(G) within a pre-determined voltage range.

In a second method step 264, the input 46 receives a signal fordetecting that the grid voltage U_(G) requires an increase in outputvoltage U_(O) from the power generating system 20, 120, i.e. in order tomaintain the grid voltage U_(G) within the desired voltage range. Thedetermining module 50 is arranged to monitor the input signal anddetermine whether the grid voltage U_(G) has dropped to a level whichcompromises the power output capability of the wind turbine 10 accordingto a predefined PQ chart. The characteristic drop in grid voltage isrepresented by the AU as illustrated by the schematic graph shown inFIG. 13.

The transformer module 54 is then configured, in a third method step266, to operate the tap changer 40 to tap up the transformer 36 in orderto provide at least part of the voltage increase required to maintainthe grid voltage U_(G) within the predetermined range. Hence, thetransformer module 54 controls the tap changer 40 to tap back the outputvoltage U_(O) to a level which allows the power generating system 20,120 to output the required power P, Q according to the predefined PQchart without changing the converter voltage reference U_(ref). Thecontrol method 260 thereby provides increased flexibility whenconfiguring the wind turbine 10 in a grid-forming configuration toprovide PQ reference values to the electrical grid 28.

Once the tap changer 40 has been adjusted then the reference voltageU_(ref) which was used previously to control the converter assembly 30will no longer correspond to the conventional droop control, asdetermined by the determining module 50 in method step 262. The changein the tap position of the tap changer 40 effectively causes adecoupling of the converter voltage reference U_(ref) from the gridvoltage U_(G) such that the previously used converter voltage referenceU_(ref) can no longer be used to control the converter assembly 30 inorder to match the demands of the grid 28. In this way, the previousconverter voltage reference U_(ref) defines an old converter voltagereference U_(ref-old) or an un-tapped reference voltage.

Consequently, if the old converter voltage reference U_(ref-old) wereused to determine the future droop control strategy of converterassembly 30, it would likely lead to errors in the grid-formingoperation of the wind turbine 10. For example, the controller 42 mayincorrectly configure the power generating system 20, 120, based on theold reference voltage U_(ref-old) to provide voltage to the grid 28based on an inaccurate understanding of the voltage which is beingoutputted from the high-voltage side of the transformer 36.

To address this problem, the controller 42 is configured in a furthermethod step to update the converter voltage reference U_(ref-old) basedon the tap position of the tap changer 40. To achieve this, thecontroller 42 is configured to monitor a first input signal indicativeof a tap position—or tap changer position—of the tap changer 40, and asecond input signal indicative of the old converter voltage referenceU_(ref-old). The controller 42 is then configured to determine a newreference voltage U_(ref-new) based on the first and second inputsignals.

The first input signal comprises a tap changer control signal configuredto control the tap position of the tap changer 40. The second inputsignal is received from a droop control module (not shown) of thecontroller 42, which is configured to control the converter assembly 30when the power generating system 20, 120 is arranged in a grid-formingconfiguration and when the tap changer 40 is configured in a neutralposition (i.e. not tapped up or tapped down). Accordingly, the droopcontrol module is configured to operate the power generating system asif it were not fitted with a tap changer 40, as would readily beunderstood by a person having ordinary skill in the art.

According to an exemplary aspect of the control method 260, thedetermining module 52 is configured to calculate a new voltage referenceU_(ref-new) using the following equation:

$U_{{ref} - {new}} = {\frac{n_{2} + {{\Delta n}*N_{TC}}}{n_{2}}U_{{ref} - {old}}}$

The new converter voltage reference U_(ref-new) defines the voltagereference which is recalculated following the tap changer adjustment. Inthis way, the new converter voltage reference U_(ref-new) represents a‘tap-adjusted’ converter voltage reference. Furthermore, n₂ representsthe number of windings at the high-voltage side of the transformer;N_(TC) is the tap position of the tap changer; and A n is the change inthe number of windings at the high-voltage side of the transformer 36for a given tap position.

The control method 260 is particularly applicable for the determinationof a voltage amplitude component of the converter voltage referenceU_(ref). By recalculating the voltage amplitude component, based on thetap position of the tap changer 40, the controller 42 is able to enhancethe power output capability of the power generating system 20, 120.

The droop control scheme 260 may be implemented to regulate the exchangeof active P and reactive Q power within the electrical grid, in order tocontrol the grid voltage frequency and amplitude. In this way, the droopcontrol scheme 260 is arranged to decrease the delivered active power Pwhen the grid voltage frequency increases and decrease the deliveredreactive power Q when the grid voltage amplitude increases. The droopcontrol scheme 260 can be implemented when the power generating system20, 120 is operated in an islanded-mode and also in a grid-connectedmode, as would be readily understood by the skilled person.

Fault Ride Through (FRT) Detection Scheme

The above described control scheme 260 is designed to operate the powergenerating system 20, 120 to control the voltage which is provided tothe electrical grid 28 by the wind turbine 10. With reference to thefirst method step 262 of control method 260, the demand to adjust theoutput voltage U_(O) of the power generating system 20, 120 requiresthat the controller 42 is able to determine the voltage at thehigh-voltage side of the transformer 36. This is needed in order todetermine the extent to which the output voltage U_(O) of the powergenerating system 20, 120 must be adjusted in order to meet the demandfrom the grid 28.

The presence of the tap changer 40 in the system means that thecontroller 42 is prevented from accurately determining the demand for achange in output voltage U_(O). In particular, the operation of the tapchanger 40 (i.e. the tapping up or tapping down of the tap changer 40)disguises any voltage fault ride through (FRT) from the high-voltageside to the low voltage side of the transformer 36. In this situation,the controller 42 may incorrectly configure the power generating system20, 120 to provide reactive power to the grid 28 based on an inaccurateunderstanding of the voltage being outputted from the high-voltage sideof the transformer 36.

It is known to monitor the voltage at the high-voltage side of thetransformer U_(HV) in order to detect FRT operating conditions. However,such techniques require additional voltage sensors to be accommodatedwithin the power generating system, which therefore increases the costand complexity of the wind turbine.

The controller 42 is arranged, according to an aspect of the presentinvention, to determine the voltage at the high-voltage side of thetransformer 36, without the need for additional monitoring equipment.Accordingly, the controller 42 is configured to execute a fault ridethrough detection scheme. To achieve this, the controller 42 isconfigured to monitor a first input signal indicative of a tapposition—or tap changer position—of the tap changer 40, and a secondinput signal indicative of the voltage at the low-voltage side of thetransformer 36. The controller 42 is then configured to control theoutput voltage U_(O) of the power generating system 20, 120 based on thefirst and second input signals.

The first input signal comprises a tap changer control signal configuredto control the tap position of the tap changer 40. Alternatively, thefirst input signal may include a tap changer sensor signal from a sensorwhich is configured to monitor the current tap position of the tapchanger 40. The second input signal comprises sensor data which isreceived from a voltage sensor that is arranged to monitor the voltageat the low voltage side of the transformer 36. The voltage data may bemeasured at any suitable point between the converter assembly 36 and thelow voltage side of the transformer 36, as would be readily understoodby the person having ordinary skill in the art.

According to a first FRT detection scheme, the determining module 52 isconfigured to calculate a virtual ‘low-voltage’ U_(LV-virtual) using thefollowing equation:

$U_{{LV} - {virtual}} = {\frac{n_{2} + {{\Delta n}*N_{TC}}}{n_{2}}U_{LV}}$

U_(LV) represents the measured voltage at the low voltage side of thetransformer 36 whereas the virtual low voltage U_(LV-virtual) definesthe voltage at the low voltage side of the transformer 36 if the tapchanger 40 was not installed. Put another way, the virtual low voltageU_(LV-virtual) represents the voltage at the low voltage side of thetransformer 36 when the tap changer 40 is configured in a neutral tapposition. FIG. 14 illustrates a schematic tap changer 40 of the powergenerating system 20, 120 according to the present invention. Theparameter n₂ represents the number of windings at the high-voltage sideof the transformer; N_(TC) is the tap position of the tap changer; and An is a change in the number of windings at the high-voltage side for agiven tap position.

Upon calculating the virtual low-voltage value U_(LV-virtual), thedetermining module 50 determines whether either the ‘virtual’ or‘measured’ low-voltage values (i.e. U_(LV-virtual) or U_(LV)) are withinan FRT voltage range or a continuous voltage range. The FRT voltagerange is a voltage range corresponding to a low or high voltage faultride through condition. The continuous voltage range corresponds to anormal operating condition of the power generating system 20, 120.

If the virtual low-voltage value U_(LV-virtual) is determined to bewithin the FRT voltage range and the measured low-voltage value U_(LV)is determined to be within the continuous voltage range, then thecontroller 42 is configured to generate a reactive current referencebased on the virtual low-voltage value U_(LV-virtual) and an associatedk-factor. All other control aspects of the power controller 42 aredetermined based on the normal operating condition of the powergenerating system 20, 120.

Alternatively, if both the ‘virtual’ and ‘measured’ low-voltage values(U_(LV-virtual) and U_(LV)) are determined to be within the FRT voltagerange, then the controller 42 is again configured to generate a reactivecurrent reference based on the virtual low-voltage value U_(LV-virtual)and an associated k-factor. However, in this case the controller 42 isconfigured to operate the power generating system 20, 120 according to aconventional FRT control scheme, i.e. in the same way that would beexpected for a power generating system which does not include a tapchanger device (i.e. as if it were a directly measured high-voltagetransformer voltage U_(HV)).

According to a second FRT detection scheme, the determining module 52 isconfigured to estimate a ‘high-voltage’ value U_(HV-estimate) accordingto the following equation:

$U_{{HV} - {estimate}} = {{\frac{n_{2} + {{\Delta n}*N_{TC}}}{n_{1}}U_{LV}} + {I*Z_{HV}}}$

The estimated high-voltage transformer voltage U_(HV-estimate)represents the voltage at the high-voltage side of the transformer 36 ifthe tap changer 40 was not installed in the power generating system 20,120. The parameter n₁ represents the number of windings at thelow-voltage side of the transformer 36. The parameter I represents thecurrent of the transformer 36 and Z_(HV) represents the impedance at thehigh-voltage side of the transformer 36. The remaining parameters of theequation are the same as those described for the first FRT detectionscheme.

The parameters I and Z_(HV) are each composed of real and imaginaryparts as defined by the following equations:

I=I _(active) +I _(reactive)

Z _(HV) =R _(HV) +jX _(HV)

I_(active) and I_(reactive) represent the active and reactive currentscorresponding to the transformer 36, respectively. R_(HV) and X_(HV)represent the resistance and reactance of the transformer 36,respectively.

According to an exemplary operation of the controller, the determiningmodule 52 is configured to estimate the ‘high-voltage’ valueU_(HV-estimate) according to the following simplified equation:

$U_{{HV} - {estimate}} = {{\frac{n_{2} + {{\Delta n}*N_{TC}}}{n_{1}}U_{LV}} + {I_{reactive}*X_{HV}}}$

The estimated high-voltage side transformer voltage U_(HV-estimate) isused by the controller 42 to detect whether there is an FRT conditionassociated with the power generating system 20, 120. Furthermore, theestimated high-voltage transformer voltage U_(HV-estimate) can also beused in a conventional FRT control scheme in the same way as a directlymeasured high-voltage transformer voltage U_(HV).

According to the above described method of determining the voltage atthe high voltage side of the transformer 36, the controller 42 is ableto adjust the power generating system 20, 120 based on eithervirtualised or estimated voltage data, and can thereby avoid FRT issueswhilst avoiding the need to accommodate additional monitoring equipmentwithin the wind turbine 10.

Although the first and second FRT detection schemes have been describedherein with reference to the controller scheme 260, it will beappreciated that the same FRT detection methods may also be applied in asimilar fashion to the control methods 60 and 160.

Efficiency Control Scheme

Given the deployment of wind turbines in remote off shore locations, itis considered that a local wind power plant controller of a wind turbinemay have access to more accurate weather estimation modelling and datamining capabilities than the traditional distribution system operators(DSOs) and transmission system operators.

Accordingly, the local power plant controller (PPC), such as thecontroller 42 described herein, is configured with additionalfunctionality such as estimation of wind power and distribution. Thecontroller 42 is arranged to receive information relating to how longthe wind turbine 10 will be required to run in a high-power, mid-powerand low-power configuration.

The determining module 50 is arranged to monitor a signal indicative ofan operating time corresponding to local wind speed and distribution.The determining module 50 is configured to determine time value in whichwind turbine is predicted to operate in each of the high-power,mid-power or low-power configurations. Each of the determined timevalues is matched to an active and reactive power reference P_(ref),Q_(ref) corresponding to the predicted output power of the converterassembly 36 for that power configuration.

The determined time values comprise time of day data as well as durationdata corresponding to the predicted wind conditions for the location ofthe wind turbine 10. The converter module 52 is configured to controlthe converter assembly 30 based on the determined time values in orderto achieve the optimum active and reactive power outputs P, Q for anygiven operating period of the wind turbine 10. The transformer module 54is configured to control the tap changer 40 to adjust the transformer 36in order to reach a desired voltage level based on the determined timevalues. The desired voltage levels increase the operational efficiencyof the power generating system 20, 120 for a given active and reactivepower output P, Q.

The determining module 50 is configured to produce an efficiencyprotocol based on determined time values and associated operatingparameter data. The efficiency protocol is stored on the storage device55 and can be updated by the determining module 50 based on new windcondition inputs. The controller 42 is arranged to then control thepower generating system 20, 120 according to the efficiency protocol.The primary objective of the control method is to determine the mostcost efficient operation of the power generating system 20, 120 based onthe prevailing wind conditions. In particular, the controller 42 isconfigured to determine whether it is cost effective to adjust the tapchanger 40 based on the predicted wind conditions, or whether it wouldbe preferable to maintain the tap changer 40 in its currentconfiguration.

If a high power wind condition is predicted to continue for a timeperiod which exceeds a threshold value, then the tap changer 40 isconfigured to tap up the transformer in order to achieve a moreefficient output from the power generating system 20, 120. However, ifthe high power wind condition is predicted to terminate before exceedingthe threshold time period, then the benefit of adjusting the tap changer40 is reduced such that it will be retained in its current tap position.The required voltage change is then obtained by controlling theconverter assembly 30.

The control method 360 can thereby avoid the unnecessary movement of tapchanger 40 to extend its lifetime and reduce long term maintenance costsassociated with the power generating system 20, 120. The control method360 can also apply to the operation of the converter assembly 30, whichthereby enables greater control the output voltage U_(O).

The power generating system 20, 120 is described herein as comprising asingle transformer 36. The transformer 36 may comprise a medium or ahigh voltage transformer as would be understood by the skilled person.Furthermore, the power generating system 20, 120 may comprise one ormore transformer devices without departing form the scope of the presentinvention.

Each of the processing modules 50, 52 and 54 are contained within thememory device 55 of the controller. The control protocol is also storedon the memory device 55 can be adapted by a user of the power generatingsystem 20, 120 in order to suit the preferred operating parameters of apower plant controller (PPC) of the grid 28, as would be readilyunderstood by a person having ordinary skill in the art.

The processor 44 is configured to perform the computer-implementedfunctions of the processing modules 50, 52 (e.g. performing a controlmethod as will be described in more detail below). The instructions whenexecuted by the processor 44 cause the processor 44 to performdetermining and controlling operations, including providing controlcommands to the various components of the power generating systems 20,120.

The controller 42 forms part of a central control system of the powergenerating systems 20, 120. As such, the controller 42 may beincorporated into any number of computer based control systems of thepower generating systems 20, 120. It should be appreciated by the personhaving ordinary skill in the art that the controller 42 is describedherein as being arranged in electronic data communication with thecomponents of the power generating systems 20, 120 using a wiredconnection, as illustrated by the dotted lines in FIGS. 2 and 3.However, in other exemplary arrangements, the power generating systems20, 120 may be coupled to the controller 42 via a wireless connection,such as by using any suitable wireless communications protocol known inthe art. Thus, the controller 42 may be configured to receive one ormore signals from the power generating system 20, 120, wirelessly.

1. A method of operating a power generating system for a wind turbineconnected to an electrical grid, the power generating system comprisinga power generator, a converter, a transformer and a tap changer, themethod comprising; monitoring a signal for detecting a voltage in theelectrical grid which requires an increase in output voltage from thepower generating system; determining a partial-load condition of theconverter, which corresponds to the converter being configured to outputa voltage which is substantially below its maximum output voltage; andupon determining the partial-load condition, operating the tap changerto tap down the transformer, and operating the converter to provide therequired increase in output voltage from the power generating system. 2.The method of claim 1, wherein operating the tap changer comprises:determining a minimum tap changer adjustment which will still allow theconverter to provide the required increase in output voltage; andoperating the tap changer to tap down the transformer to implement theminimum tap changer adjustment.
 3. The method of claim 1, wherein themethod comprises: monitoring a signal for detecting the speed of thewind in the area of the wind turbine; and determining an operatingcondition of the power generating system based on the detected windspeed.
 4. The method of claim 3, wherein the method comprises;determining that the power generating system is operating in a firstoperating condition; monitoring a signal for detecting an operatingcurrent I_(C) of the converter; and upon determining that the operatingcurrent I_(C) is above a threshold current I_(T), inhibiting the tapdown of the transformer.
 5. The method of claim 4, wherein the thresholdcurrent I_(T) is 1.0 per unit system.
 6. The method of claim 4, whereinthe first operating condition corresponds to the wind speed beingbetween 0 m/s and 10 m/s.
 7. The method of claim 3, wherein the methodcomprises: determining that the power generating system is operating ina second operating condition; and operating the tap changer to limit themagnitude of the tap-down of the transformer based on the detected windspeed.
 8. The method of claim 7, wherein the second operating conditioncorresponds to a wind speed of between 10 m/s and 11 m/s.
 9. The methodof claim 3, wherein the method comprises disabling the method ofoperating the power generating system upon determining that the windspeed is above a pre-determined wind threshold.
 10. The method of claim1, wherein the power generating system is configured to provide anover-voltage to the electrical grid; the method comprising, upondetermining that the power generating system is no longer required toprovide an overvoltage to the electrical grid, operating the converterand the tap changer according to a pre-determined operating protocol.11. The method of claim 1, wherein monitoring the signal for detecting avoltage in the electrical grid which requires an increase in outputvoltage from the power generating system comprises; monitoring a firstsignal indicative of a tap position of the tap changer; monitoring asecond signal indicative of a voltage at a low-voltage side of thetransformer; and determining the voltage of a high-voltage side of thetransformer based on the first and second signal.
 12. The method ofclaim 11, wherein determining the voltage at the high-voltage side ofthe transformer comprises:$U_{{LV} - {virtual}} = {\frac{n_{2} + {{\Delta n}*N_{TC}}}{n_{2}}U_{LV}}$wherein U_(LV-virtual) defines a virtual voltage at the low-voltage sideof the transformer when the tap changer is configured in a neutralposition, n₂ is a number of windings at a high-voltage side of thetransformer, N_(TC) is the tap position of the tap changer, Δn is achange in the number of windings at the high-voltage side for a giventap position, and U_(LV) defines the actual voltage at the low-voltageside of the transformer (36); wherein an over-voltage condition isdetected if both the virtual voltage U_(LV-virtual) and the actualvoltage U_(LV) are determined to be within an over-voltage range. 13.The method of claim 11, wherein determining the voltage at thehigh-voltage side of the transformer comprises:$U_{{HV} - {estimate}} = {{\frac{n_{2} + {{\Delta n}*N_{TC}}}{n_{1}}U_{LV}} + {I_{reactive}*X_{HV}}}$wherein U_(HV-estimate) defines an estimated voltage at the high-voltageside of the transformer when the tap changer is configured in a neutralposition, n₂ is a number of windings at a high-voltage side of thetransformer, n₁ is a number of windings at the low-voltage side of thetransformer, N_(TC) is the tap position of the tap changer, Δn is achange in the number of windings at the high-voltage side for a giventap position, U_(LV) defines the actual voltage at the low-voltage sideof the transformer, I_(reactive) is a reactive current of thetransformer, and X_(HV) is an impedance of the transformer; wherein anover-voltage condition is detected if the estimated voltageU_(HV-estimate) is determined to be within an over-voltage range.
 14. Acontroller for controlling a power generating system comprising a powergenerator, a generator side converter, a grid side converter, atransformer, a tap changer for a wind turbine, the controller beingarranged to be connected to the power generating system and configuredto control the power generating system according to an operation ofoperating a power generating system for a wind turbine connected to anelectrical grid, the power generating system comprising a powergenerator, a converter, a transformer and a tap changer, the operationcomprising; monitoring a signal for detecting a voltage in theelectrical grid which requires an increase in output voltage from thepower generating system; determining a partial-load condition of theconverter, which corresponds to the converter being configured to outputa voltage which is substantially below its maximum output voltage; andupon determining the partial-load condition, operating the tap changerto tap down the transformer, and operating the converter to provide therequired increase in output voltage from the power generating system.15. A power generating system for a wind turbine which is connected toan external electrical grid, the power generating system comprising aconverter, a transformer, a tap changer, and a controller, thecontroller comprising; an input arranged to receive an input signalindicative of a voltage of the electrical grid; a determining modulearranged to determine a demand for an increase in output voltage fromthe power generating system based on the input signal; a transformercontrol module arranged to determine a transformer control signal tooperate the tap changer to tap-down the transformer; a converter controlmodule arranged to determine a converter control signal to operate theconverter; and an output arranged to transmit the converter andtransformer control signals to the power generating system; wherein thedetermining module is configured to determine a partial-load conditionof the converter which corresponds to the converter being configured tooutput a voltage which is substantially below its maximum outputvoltage, and upon determining the partial-load condition, thetransformer control module is configured to operate the tap changer totap-down the transformer, and the converter control module is configuredto operate the converter to provide the required increase in outputvoltage from the power generating system.